An object of value according to the invention may be any object to be protected, for example proprietary articles or documents of value. Objects of value according to the present invention are in particular bank notes, but also shares, certificates, postage stamps, checks, check cards, credit cards, ID cards, passports, admission tickets, travel tickets, plane tickets and the like, as well as labels, seals, packages or other elements for product protection. The simplifying designation “object of value” or “security element” will therefore hereinafter always include documents of the stated kind.
It has been known for some time to use thermochromic materials for protecting documents of value. For example, DE 22 12 350 describes a security thread of transparent plastic that has cavities. Said cavities contains a liquid-crystalline material that shows a reversible color change upon an increase or decrease in temperature.
EP 0 608 078 B1 likewise discloses a security thread with thermochromic properties. In this case, a plastic material is provided with a print or with characters resulting from partially demetalizing a metal layer. Disposed above said print or negative characters is a thermochromic coating that is colored at normal temperature. When heated, the thermochromic coating turns colorless so that the characters therebelow become recognizable. Alternatively, one can also use a thermochromic coating that is colorless at normal temperature and turns colored when heated so that the characters disappear. This thread is incorporated into the security paper so as to pass directly to the surface in certain areas, so-called “windows.”
Such thermochromic security threads have the disadvantage, however, that the thermochromic effect is used for purely decorative purposes in many areas not involving security, so that the optical effect produced by the color change of the thermochromic material is not recognized by the viewer as a security feature but merely as a design variant. Such security elements thus do not offer high protection from forgery.